The invention relates to micromechanical elements deflectable in an oscillating manner and to a method for the operation of such elements.
Spring-mass oscillators are frequently used as oscillators in micromechanics. In this connection, movably supported elements having mass are held by one or more spring elements. The spring elements effect restoring forces in the direction of the position of rest of the deflected elements. The oscillating deflection between two reversal points can be initiated using electrical AC voltage. In this process, the resonant frequency of an element deflected in an oscillating manner can be taken into account and utilized for a maximum deflection achievable with a reduced performance.
The drive therefore frequently takes place at least in the proximity of a resonant frequency.
This is problematic, for example, with a drive concept which is called an “out-of-plane electrode comb” and is described by H. Schenk in the following German language publication: “An innovative microactuator for the one-dimensional and two-dimensional deflection of light”; dissertation 2000; Gerhard-Mercator University Duisburg. In this respect, in addition to a hysteresis effect, further disadvantages also occur. For instance, on the presence of spring elements with linear spring characteristics for the oscillating deflection, only electrical AC voltages can sensibly be used with frequencies larger than the resonant frequencies of the elements to be deflected. This substantially increases the effort and/or cost for the control of such a deflection since the oscillation collapses at a frequency below the resonant frequency.
In the operation of these known elements, a procedure is followed such that the oscillation is started by means of a sequence of voltage pulses having a frequency of a magnitude of the fourfold of the mechanical resonant frequency. In regulated operation, an electrical AC voltage is then used having a frequency corresponding to double the resonant frequency.
It is to be noted that the maximum oscillation amplitude cannot be reached if the frequency is increased starting from lower values. This has the result that with an oscillation with a maximum amplitude of the deflection, every ever so small reduction in the excitation frequency results in the collapse of the oscillation. Accordingly, a new “start-up” with a substantially higher excitation frequency must be carried out as mentioned above.
An exact regulation of frequency and phase is therefore required for stable operation under resonant conditions. Only in this way can work be carried out in stable fashion with maximum deflection. A correspondingly high effort and/or cost for the regulation is necessary.
If this is not reached, or if it cannot be reached, no utilization of the maximum possible deflection is present since, as mentioned, slight deviations from the preset excitation frequency (namely from double the resonant frequency) can result in the breakup of the oscillation. The deflection must therefore be limited to secure a stable operation so that a maximum possible deflection cannot be utilized.
A further point to be considered is the stability of the amplitude of the deflection. It likewise depends on the excitation frequency in the proximity of the resonant frequency. A small change in the excitation frequency in the proximity of the resonant frequency can thus result in an increased rise in the amplitude.
These relationships can be better understood with the diagrams shown in FIGS. 2 and 3.
This maximum achievable deflection is in particular limited with a small length and linear spring characteristics of the spring elements used which should be operated at very high frequencies. An element can thus be held by two straight-line torsion spring elements. As the deflection increases, that is, with larger angles of rotation, the torsion spring elements become stiffer and the spring characteristics are then progressive. In conjunction with the previously described disadvantages and with a non-regulated operation, this has the result that the maximum possible deflection cannot be reached. The resonant frequency increases as the deflection rises due to the progressive spring behavior. The frequency response therefore starts to reverse, as can be seen from the diagram shown in FIG. 3. The frequency retardation on start-up would have to be reversed from a specific amplitude/frequency ratio onward, which is not possible in reality in a non-regulated operation.
The resonant frequency is usually a base parameter for the design of oscillating elements/systems which should/can be operated at resonance. Only slight deviations are permitted. It is therefore endeavored to keep the dependence of the frequency on other parameters such as the respective deflection as small as possible. Spring elements are therefore used with linear spring characteristics to avoid a changing resonant frequency.